This invention relates to a novel design for an optical fiber, and in particular to designs for optical fibers that provide a large core single-mode fiber for ultra-high capacity transmission.
The most effective way of increasing the transmission capacity of an optical fiber communication system is to use the extremely wide bandwidth of an optical fiber and to feed a large number of channels as is practicable into the fiber. As a consequence of this, the trend has been to extend the communication window from the C-Band to the L-Band and the S-Band so that the communication window covers a total range of about 200 nm.
However, a major obstacle to the development of ultra-wide-band dense wavelength division multiplexing systems are non-linear effects, and in particular the Raman effect, which cannot be managed with dispersion. A direct way to manage such non-linear effects is to use a fiber with a large effective core area, and therefore recently much work has been done on developing such fibers.
In 1996 Corning Inc. developed a large-effective-area dispersion-shifted fiber (LEAF) which had an effective core area of about 80 xcexcm2, and there are other designs for large-effective-area fibers, one of which shows an effective area of about 100 xcexcm2. A single material photonic crystal fiber, a so-called holey fiber, is characterized by a distribution of air holes in the cladding running through the entire length of the cladding and has attracted considerable attention in recent years because it is capable of single-mode operation over a wide range of wavelengths. However, it is difficult to keep the birefringence, and hence the polarization mode dispersion in the fiber, low because of the large index contrast introduced by the air holes.
According to the present invention there is provided an optical fiber comprising a central core region surrounded by an annular cladding region, wherein said cladding region is formed with a refractive index that varies periodically in the azimuthal direction.
In a preferred embodiment of the invention the cladding region comprises a plurality of alternating segments. In a first and particularly simple embodiment of the invention, the segments may include a first set of segments having a first refractive index alternating with a second set of segments having a second refractive index. The first refractive index may be higher than said second refractive index, and the first refractive index is equal to or less than the refractive index of said core region.
Alternatively, however, as opposed to providing all the segments as step-index segments with a specific refractive index, in other embodiments of the invention at least some of the segments may be formed with a graded index, such that they have a refractive index that varies radially and/or azimuthally. In one preferred embodiment, for example, the refractive index of all the segments is graded azimuthally whereby said cladding region varies smoothly in the azimuthal direction.
In other possible embodiments the duty cycle of the azimuthal periodic variation may be either constant in the radial direction or may vary in the radial direction by selecting the shape of the segments.
According to another broad aspect of the invention there is further provided an optical fiber comprising a central core region surrounded by an annular cladding region, wherein said cladding region is formed with a refractive index that varies periodically in the azimuthal direction, wherein said cladding region is formed of a plurality of segments at least some of which are formed with a graded refractive index.